The present invention relates to an optical pickup for correcting an astigmatic difference of light emitted from a light source.
Generally, an optical pickup used for recording/reproducing information on/from a recording medium using a laser beam includes, as shown in FIG. 1, a light source 10 for generating and emitting a laser beam, a collimating lens 20 for collimating the light of the laser beam emitted from the light source 10, a light path changing means 30 for changing the traveling path of an incident light, an objective lens 40 for converging the light from the light source 10 to form a light spot on a recording surface of a recording medium 1, and a photodetector 60 for detecting an information signal and an error signal from the light reflected from the recording medium 1. Also, the optical pickup may include a cylindrical lens 50 along the optical path between the light path changing means 30 and the photodetector 60, for detecting a focus error signal by an astigmatism method. In this example, a small, light edge emitting laser diode is adopted as the light source 10.
However, as shown in FIG. 2, the edge emitting laser diode emits an elliptical light beam from an active layer 11 thereof. This is due to the difference in source points 12a and 12b of the light beam, wherein distance (.DELTA.Z) between the source points 12a and 12b is defined as the "astigmatic difference". Accordingly, the collimating lens 20 cannot be aligned properly with respect to both source points 12a and 12b.
In order to solve the above problems, a conventional optical pickup adopts, as shown in FIGS. 3A and 3B, a pair of first and second cylindrical lenses 21 and 22 having different focal distances as a collimating lens 120. FIG. 3A is a diagram showing the first and second cylindrical lenses 21 and 22 viewed from the Y-axis (see FIG. 2), i.e., the thickness direction of the active layer 11, and FIG. 3B is a diagram showing the first and second cylindrical lenses 21 and 22 viewed from the X-axis, i.e., the width direction of the active layer 11.
Assuming that the focal distance of the first cylindrical lens 21 is f.sub.1, and that of the second cylindrical lens 22 is f.sub.2, the relationship of a diameter W.sub.o of light which has passed through the first and second cylindrical lenses 21 and 22 and the diameter W.sub.i of light which has not passed through the first and second cylindrical lenses 21 and 22 is defined as follows. ##EQU1##
Accordingly, the diameter of the light beam along the X-axis can be matched to that of the light beam along the Y-axis through adjustment of the diameter of the beam in the X-axial direction by the first and second cylindrical lenses 21 and 22.
However, there are difficulties in manufacturing a lens having excellent wavefront aberration and adjusting the optical axis for the above method.
Another conventional optical pickup for correcting an astigmatic difference further includes a prism 23 shown in FIG. 4 together with the collimating lens 20 shown in FIG. 1. Assuming that the incident light angle of a light upon a receiving surface of the prism 23 is .theta..sub.i and the emitting angle of a light from an emitting surface thereof is .theta..sub.o, the relationship of a diameter W.sub.o of light which has passed through the prism 23 and the diameter W.sub.i of light which has not passed through the prism 23 is defined as follows. ##EQU2##
Accordingly, the astigmatic difference of the light emitted from the light source can be corrected with the use of the prism.
However, in the conventional optical pickup which further includes a prism 23 together with a collimating lens 20, if there is a tilt or shift in the light path, the assembly process becomes complicated. Also, since the optical path is elongated, it is difficult to manufacture a compact optical pickup.
In another conventional optical pickup, as shown in FIG. 5, a flat glass 24 between the light source 10 and the collimating lens 20 (see FIG. 1) is further included. The flat glass 24 is arranged at an angle with respect to the emitted light and integrally formed with a housing 25 protecting the light source 10. However, a coma aberration occurs due to the coupling of the flat glass 24 and the collimating lens 20.